not that I know of.. however, if you feel really adventuruos, you could create a new color, using the same tiles and script as the forest, and just use it sparingly inside of the "real" forest. Remove the yew tree from the "real" forest and put it in the "new" forest.
that would do two things: it would keep the yew trees from popping up all over and also give you more control as to where they go.
Here's a basic example of what I mean:
The dark green would be grass, the meduim green is the "real" forest, and the light green is a new color for "new" forest. You'd make the script for the new forest the same as the real forest, cept adding the yew tree part into it. You'd be able to control what are of the forest has Yew trees this way, as I didn in the example pic, I only have the area on the left.
If you'd like to pursue this, let me know. depending on how hilly the forest is would depend on how many new colors you need, and maybe losing some colors you don't use.
Let me know if you are interested in pursuing this and I'll help you as I can.
-Ryandor